My ILs are a bundle of dysfunction, but apart from the actual dysfunction, I can't actually think of anything I'd class a weird that they do, which has me a bit stumped really.
Let me tell you something weird my mum does though that freaks me out. I am NC with my mum now, but I'm sure she still does it. Whenever she goes anywhere away overnight, she preps however many days worth of food and snacks for my stepdad. Like literally every single item of food that he eats parcelled out into little pots for him. She'll pre-cook jackets and add the toppings, salads made, he gets several 'treat' days of takeaways, which she orders in advance, then puts into little pots for him, Tuesday is Chinese, Wednesday is pizza, oh for Friday, he might get KFC. He is a perfectly functional adult. He's retired now, but until recently he was working FT in hedge funds. He knows how to make a jacket or to order takeaway.
It sounds like it's because he's controlling and has these sort of trad wife expectations, but he doesn't. I'm not sure he even cares. But the thing is that I don't think he would ever eat anything she didn't package up for him. Like she sets the feeding schedule for the week and he sticks to it. When they used to visit us (before NC), she would fix all his plates for him. Then when he was done, he'd go around the table asking for people's leftovers, even while they were still eating. It was like he wouldn't get a second helping because she wouldn't dish it up for him, even when we all were helping ourselves. So he'd ask to eat so and so's roast potato or bit of gammon or whatever. It's weird. Neither of them are 'controlling' but they are dysfunctionally enmeshed in a weird way, so I think they must rely on each other to make food related decisions.
They'd also often order like two starters and a main when eating out, and then should share the food between them, dishing certain amounts to each others plates. That on it's own, maybe not weird, and seems cost efficient (they are literal millionnaires, so money is no object), but in the bigger picture of the weird food issues, it's just odd to me. It used to drive me bonkers. I wanted to scream, could you please just order food and eat like the rest of us!! Because every meal out would be this negotiation about what collective dishes they wanted and then having to explain it to the wait staff and ask for little side plates. Made me want to hide under the table sometimes.